I've been reading a bit and angelfish does seem to be about right. Like I said I don't want something too extreme! I also don't want anything that is going to be too fussy about water quality so I suppose cichlids are out. If I can just stop them reproducing it would be a start!

Spend a bit extra and get ones in the 2-3" range, they take ages to grow on 'normal' fishfood, then they'll go straight after the fry and smaller guppies.
Plus, by that age, the ones with weaker immune systems have already died off. If you do go for the smaller (cheaper) ones though, they'll only be able to eat the fry and expect a survival ratio of about 1 in 4. They've been inbred too much in the trade and weak fish are the norm.

If you do go for the smaller (cheaper) ones though, they'll only be able to eat the fry and expect a survival ratio of about 1 in 4. They've been inbred too much in the trade and weak fish are the norm.

That's an interesting comment, and goes a long way towards explaining the problems we had with them - tumours, deformities, swim bladder abnormalities, etc, all of which developed long term as the fish grew.

As I've said in other posts, I have bred many fish, Angels being one such species (egg eating swines!!!) and trying to breed from regular stock, especially when bought young (and cheap) is a waste of time as far as I'm concerned. It works out cheaper to buy a pair of good sized Angels that you can be fairly certain have passed the developmental stages than buying 8 juveniles, most of which will bloody die! And in doing so possibly affect the water quality.

Ended up buying a pair of F2's for my breeding project, from different places to avoid inbreeding and the resulting mess that follows. Bigger ones are also much easier to sex.

I still recommend them as guppy hunters, despite the drawbacks and on a regular diet of live food, you'll be unlucky if a pair DOESN'T breed in good water conditions and pH6-8.4. Don't worry, they'll eat their own eggs unless you remove the site (usually a smooth slate).
I used Platy fry to condition my pair.

One of the girls has a tank in the corner of the living room that I seem to have inherited responsibility for. The "interesting" fish have long since expired and now it contains nothing but hundreds of guppies. What can I put in there to thin them down a bit? Preferably something that will start on the babies and work its way up as it gets bigger in a not too horrifically quick and attention drawing manner (so don't say an oscar!)

Guppies. Adult guppies have to be separated from their off spring when they are born so they don't eat them all.